Semiconductor laser

ABSTRACT

A semiconductor laser includes semiconductor layers stacked on a substrate, and a pair of resonator end surfaces opposed to each other in the direction perpendicular to the stacking direction. In this semiconductor laser, a light emission side reflecting film is formed on one of the resonator end surfaces. A refractive index of the reflecting film against an emission wavelength of laser light is set to a value between an effective refractive index and a refractive index of the substrate. Another semiconductor laser includes a light emission function layer stack including a cladding layer and an active layer formed on one place of a translucent substrate; two electrodes having different polarities, which are provided on the light emission function layer stack side; and a light leakage preventive film formed on the other plane of the translucent substrate.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor laser includingsemiconductor layers stacked on a substrate and a pair of resonator endsurfaces opposed to each other in the direction perpendicular to thestacking direction.

In recent years, a semiconductor laser (laser diode: LD) has been usedfor various optical systems. In general, a semiconductor laser has astructure in which a first conductive type semiconductor layer, anactive layer, and a second conductive type semiconductor layer aresequentially stacked on a substrate, wherein light generated from theactive layer is amplified between a pair of resonator end surfacesopposed to each other in the direction perpendicular to the stackingdirection. In many cases, reflecting films for adjusting the reflectanceand protecting the resonator end surfaces are provided on the pair ofresonator end surfaces. The reflecting film on the side from which laserlight is mainly emitted is adjusted such that the reflectance thereofbecomes lower, and the reflecting film on the non-light emission side isadjusted such that the reflectance thereof becomes higher.

These reflecting films are generally each configured to have a singlelayer structure or a multi-layer structure depending on the applicationof the semiconductor laser. In particular, the light emission sidereflecting film is often configured to have a single layer structurefrom the viewpoint of simplicity of film formation. For example, in asemiconductor laser in which nitride based group III-V compoundsemiconductor layers are stacked on a sapphire substrate, a reflectingfilm on the light emission side generally has a single layer structureof aluminum oxide (Al₂O₃) or silicon oxide (SiO₂) having a refractiveindex against an emission wavelength, which is smaller than that of thestack of the nitride based group III-V compound semiconductor layers.

The reflecting film made from aluminum oxide or silicon oxide, however,has a problem. Namely, the refractive index of the reflecting filmagainst an emission wavelength, which is smaller than that of thenitride based group III-V compound semiconductor layers as describedabove, also becomes smaller than that of the substrate. Accordingly, asshown in FIG. 11, if the thickness of the reflecting film is adjustedsuch that the reflectance in a region, corresponding to the nitridebased group III-V compound semiconductor layers, of the reflecting filmbecomes lower, the reflectance in a region, corresponding to thesubstrate, of the reflecting film also becomes lower. In addition, FIG.11 shows a relationship between a thickness of a reflecting film and areflectance of the reflecting film against an emission wavelength of 400nm for a semiconductor laser in which nitride based group III-V compoundsemiconductor layers are stacked on a sapphire substrate and areflecting film made from aluminum oxide is formed on the light emissionside. In the figure, a solid line designates the reflectance in aregion, corresponding to the substrate, of the reflecting film and abroken line designates the reflectance in a region, corresponding to anactive layer (an oscillation region), of the reflecting film.

Further, since the sapphire substrate is translucent against an emissionwavelength, if the semiconductor laser is used while being contained ina package, stray light reflected in the package may enter thesemiconductor laser from the region, corresponding to the substrate, ofthe reflecting film. As a result, there arises a problem in causingnoise and thereby degrading the characteristics of the semiconductorlaser.

The present invention also relates to a semiconductor laser includingvarious function films provided on a translucent substrate such as asapphire substrate.

A semiconductor laser using a translucent substrate such as a sapphiresubstrate has a problem that much of laser light is leaked through thesapphire substrate as spontaneous emission light. The spontaneousemission light may be emerged to the outside of a package in which thesemiconductor laser is contained, to exert adverse effect on peripheralparts.

In particular, for the next-generation optical pickup in which a storagemedium (disk) is closer to a semiconductor laser, the above-describedspontaneous emission light may become a serious problem because itcauses noise.

From the viewpoint of preventing the leakage of such spontaneousemission light, Japanese Patent Laid-open No. Hei 11-87850 discloses asemiconductor laser in which a light absorbing layer having a band gapsmaller than that of an active layer is provided in a stack ofsemiconductor layers.

From the viewpoint of increasing an output efficiency of light, JapanesePatent Laid-open No. Hei 11-17223 discloses a semiconductor laser inwhich a reflecting layer for reflecting light emitted from an activelayer is provided in a stack of semiconductor layers.

The above-described related art semiconductor lasers, however, haveproblems. Namely, since the light absorbing layer for preventing leakageof spontaneous emission light or the reflecting layer for increasing theoutput efficiency of light is built in the stack of semiconductorlayers, it is difficult to set a light absorbing condition, a lightreflecting condition, and a film formation condition because suchconditions must be set in consideration of the relationship withadjacent other films. Further, since the light absorbing film or thelight reflecting layer is provided in the stack of semiconductor layers,the original characteristics of the semiconductor laser may be degraded,thereby failing to achieve a desired performance of the semiconductorlaser.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a semiconductorlaser capable of suppressing stray light from entering the semiconductorlaser from a region, corresponding to a substrate, of a light emissionside reflecting film, thereby improving the characteristics of thesemiconductor laser.

A second object of the present invention is to provide a semiconductorlaser using a translucent substrate, which is capable of suppressingleakage of spontaneous emission light.

To achieve the first object, according to the first invention, there isprovided a semiconductor laser including a substrate, semiconductorlayers stacked on the substrate, and a pair of resonator end surfacesopposed to each other in the direction perpendicular to the stackingdirection, the semiconductor laser including: a light emission sidereflecting film formed on one of the resonator end surfaces; wherein arefractive index of the reflecting film against an emission wavelengthof laser light is set to a value between an effective refractive indexand a refractive index of the substrate.

With this configuration, the reflective index of the reflecting filmagainst an emission wavelength of laser light is set to a value betweenthe effective refractive index and a refractive index of the substrate,and accordingly, if the thickness of the reflecting film is adjustedsuch that the reflectance in a region, corresponding to an oscillationregion of laser light, of the reflecting film becomes lower, thereflectance in a region, corresponding to the substrate, of thereflecting film becomes higher, with a result that stray light enteringthe semiconductor laser from the region, corresponding to the substrate,of the reflecting film can be reduced.

To achieve the second object, according to the second invention, thereis provided a semiconductor laser including: a light emission functionlayer stack including a cladding layer and an active layer formed on oneplane of a translucent substrate; two electrodes having differentpolarities, which are provided on the light emission function layerstack side; and a light leakage preventive film formed on the otherplane of the translucent substrate.

To achieve the second object, according to the second invention, thereis also provided A semiconductor laser including: a light emissionfunction layer stack including a cladding layer and an active layerformed on one plane of a translucent substrate; and an electrode servingas light leakage preventive film for shielding light and injecting acurrent in the light emission function layer stack, which is formed onthe other plane of the translucent substrate.

With these configurations, the light leakage preventive film is formedon the surface, opposed to the light emission function layer stack, ofthe translucent substrate, and accordingly, even if laser lightgenerated from the light emission function layer stack is leaked to thetranslucent substrate side, it is absorbed or reflected in or from thelight leakage preventive film, to thereby suppress leakage of light tothe outside. Further, since the light leakage preventive film is formedon the side, opposed to the light emission function layer stack, of thetranslucent substrate, it is possible to prevent the light leakagepreventive film from exerting an adverse effect on the laser lightgeneration characteristic by the light emission function layer stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial exploded perspective view showing a configuration ofa semiconductor laser according to one embodiment of the firstinvention;

FIG. 2 is a characteristic diagram showing a relationship between athickness and a reflectance of a reflecting film of the semiconductorlaser shown in FIG. 1;

FIG. 3 is a schematic sectional view illustrating a structure of asemiconductor laser according to an embodiment of the second invention;

FIG. 4 is a graph illustrating a relationship between a current and anoutput of light;

FIG. 5 is a graph illustrating a reflectance of the light leakagepreventive film having a two-layer structure;

FIG. 6 is a graph illustrating a reflectance of the light leakagepreventive film having a four-layer structure;

FIG. 7 is a graph illustrating a reflectance of the light leakagepreventive film having a six-layer structure;

FIG. 8 is a graph illustrating a reflectance of the light leakagepreventive film having an eight-layer structure;

FIG. 9 is a schematic sectional view illustrating another embodiment ofthe second invention;

FIG. 10 is a schematic sectional view, similar to FIG. 1, of asemiconductor laser in which the second invention is combined with thefirst invention; and

FIG. 11 is a characteristic diagram illustrating a relationship betweena thickness and a reflectance of a reflecting film of a related artsemiconductor laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment of a first invention will bedescribed in detail with reference to the drawings.

FIG. 1 shows a configuration of a semiconductor laser according to theembodiment of the first invention. The semiconductor laser includes asemiconductor layer stack 20 on one plane of a substrate 11. Each layerof the semiconductor layer stack 20 is composed of a nitride based III-Vcompound semiconductor containing at least one kind of group IIIAelements and at least nitrogen (N) of group VA elements in theshort-period type periodic table. The semiconductor layer stack 20 has abuffer layer 21, an n-side contact layer 22, an n-type cladding layer23, an n-type guide layer 24, an active layer 25, a p-type or n-typeguide layer 26, a p-type cladding layer 27, and a p-side contact layer28, which layers are sequentially stacked in this order on the substrate11. Of these layers, the n-side contact layer 22, the n-type claddinglayer 23, and the n-type guide layer 24 or 26 are n-type semiconductorlayers which correspond to first conductive type semiconductor layers,while the p-type guide layer 26, the p-type cladding layer 27, and thep-side contact layer 28 are p-type semiconductor layers which correspondto second conductive type semiconductor layers.

The substrate 11 is made from sapphire having a thickness in thestacking direction (hereinafter, referred to simply as “thickness”) of90 μm. The semiconductor layer stack 20 is formed on the c-plane of thesubstrate 11.

The buffer layer 21, which has a thickness of 30 nm, is made fromundoped GaN. The n-side contact layer 22, which has a thickness of 3 μm,is made from n-type GaN doped with silicon (Si) as an n-type impurity.The n-type cladding layer 23, which has a thickness of 1 μm, is madefrom n-type AlGaN mixed crystal doped with silicon as an n-typeimpurity. The n-type guide layer 24, which has a thickness of 0.1 μm, ismade from n-type GaN or n-type InGaN doped with silicon as an n-typeimpurity.

The active layer 25, which has a thickness of 30 nm, has a multi-quantumwell structure formed by stacking mixed crystal layers to each other.The mixed crystal layers are made from Ga_(X)In_(1−x)N (1≧x≧0), whereinthe value of x differs for each mixed crystal layer. At least part ofthe active layer 25 functions as a light emitting portion. Thewavelength of light emitted from the light emitting portion is typicallyset to about 400 nm.

The p-type guide layer 26 or n-type guide layer 26, which has athickness of 0.1 μm, is made from p-type GaN or InGaN doped withmagnesium (Mg) as a p-type impurity. The p-type guide layer 26 may bereplaced with the n-type guide layer 26 which may be made from undopedGaN or n-type GaN doped with silicon (Si) as an n-type impurity. Thep-type cladding layer 27, which has a thickness of 0.8 μm, is made fromp-type AlGaN mixed crystal doped with magnesium as a p-type impurity.The p-side contact layer 28, which has a thickness of 0.5 μm, is madefrom p-type GaN doped with magnesium as a p-type impurity. In addition,part of each of the p-side contact layer 28 and the p-type claddinglayer 27 is formed into a narrow strip shape extending in a resonatordirection A perpendicular to the stacking direction of the semiconductorlayer stack 20. Accordingly, the semiconductor laser in this embodimentenables current constriction, thereby allowing a region, correspondingto the p-side contact layer 28, of the active layer 25 to function asthe light emitting portion.

In this semiconductor laser, the width of the n-side contact layer 22 inthe direction perpendicular to the resonator direction A is set to bewider than the width of each of the n-type cladding layer 23, the n-typeguide layer 24, the active layer 25, the n-type guide layer 26, thep-type cladding layer 27, and the p-side contact layer 28, and thesen-type cladding layer 23, n-type guide layer 24, active layer 25, n-typeguide layer 26, p-type cladding layer 27, the p-side contact layer 28are stacked on part of the n-side contact layer 22.

An insulating film 31 made from silicon dioxide is formed to cover aregion from the surface of the n-side contact layer 22 to the surface ofthe p-side contact layer 28. Openings are provided in the insulatingfilm 31 at positions over the n-side contact layer 22 and the p-sidecontact layer 28. An n-side electrode 32 is formed on the n-side contactlayer 22 via the n-side contact layer side opening, and a p-sideelectrode 33 is formed on the p-side contact layer 28 via the p-sidecontact layer side opening. The n-side electrode 32 is formed bysequentially stacking a titanium (Ti) layer and an aluminum (Al) layeron the n-side contact layer 22, and alloying the metals of these layersby heat-treatment. The n-side electrode 32 is electrically connected tothe n-side contact layer 22. The p-side electrode 33 is formed bysequentially stacking a palladium (Pd) layer, a platinum (Pt) layer, anda gold (Au) layer on the p-side contact layer 28. The p-side electrode33 is electrically connected to the p-side contact layer 28.

The semiconductor layer composed of the semiconductor layer stack 20 andthe substrate 11 has a pair of resonator end surfaces 41 and 42 opposedto each other in the resonator direction A. A light emission sidereflecting film 43 is formed on one resonator end surface 41, and anon-light emission side reflecting film 44 is formed on the otherresonator end surface 42. The reflecting film 43 is adjusted such thatthe reflectance thereof against an emission wavelength in a regioncorresponding to an oscillation region of laser light becomes lower, andthe reflecting film 44 is adjusted such that the reflectance thereofagainst an emission wavelength in a region corresponding to theoscillation region of laser light becomes higher. With thisconfiguration, laser light generated by the active layer 25 and itsneighborhood is amplified between the reflecting films 43 and 44 and ismainly emitted from the reflecting film 43 side. In addition, a laserbeam is emitted even from the reflecting film 44 side depending on thereflectance thereof, although the quantity of the laser beam on thereflecting film 44 side is very much smaller than that on the reflectingfilm 43 side; however, in this specification, one reflecting film sidefrom which a laser beam is intended to be emitted is called “lightemission side” and the other reflecting film side is called “non-lightemission side”.

The reflecting film 43 contains at least one kind of aluminum nitride(AlN), zirconium oxide (ZrO₂), and silicon oxynitride (SiO_(x)N_(y)).Against an emission wavelength (typically, 400 nm) of laser light, therefractive index of the reflecting film 43 is set to a value between arefractive index of the substrate 11 and an effective refractive index.It is to be noted that the effective refractive index means an averagerefractive index in an oscillation region of laser light, and that theoscillation region of laser light is mainly the active layer 25 andfurther contains its neighborhood, for example, the n-type guide layer24, the p-type guide layer 26, and part of the n-type cladding layer 23,and part of the p-type cladding layer 27.

According to this embodiment, against an emission wavelength of 400 nm,the refractive index of the substrate 11 is set to 1.77 and theeffective refractive index is set to 2.5, and the refractive index ofthe reflecting film 43 is set to be larger than the refractive index ofthe substrate 11 and is smaller than the effective refractive index.With respect to the refractive index of the reflecting film 43(containing at least one kind of aluminum nitride, zirconium oxide, andsilicon oxynitride) against an emission wavelength of 400 nm, therefractive index of aluminum nitride is 2.13, the refractive index ofzirconium oxide is 2.07, and the refractive index of silicon oxynitridevaries in a range of 1.5 to 2.07 depending on the contents of oxygen (O)and nitrogen (N).

The reason why the refractive index of the reflecting film 43 isspecified as described above is as follows: namely, in the case wherethe thickness of the reflecting film 43 is adjusted such that thereflectance thereof in the region corresponding to the oscillationregion of laser light becomes lower, the reflectance thereof in theregion corresponding to the substrate 11 can be made higher, to therebysuppress stray light from entering the semiconductor laser. Inparticular, it is preferred that the refractive index of the reflectingfilm 43 is set to a nearly intermediate value between the refractiveindex of the substrate 11 and the effective refractive index. In thiscase, when the reflectance in the region, corresponding to oscillationregion of laser light, of the reflecting film 43 is minimized, thereflectance in the region, corresponding to the substrate 11, of thereflecting film 43 can be substantially maximized.

FIG. 2 is a graph showing a relationship between the thickness of thereflecting film 43 and the reflectance thereof against an emissionwavelength of 400 nm. In addition, the reflecting film 43 used here isformed of a single layer film of aluminum nitride. In this figure, asolid line indicates a reflectance in a region, corresponding to thesubstrate 11, of the reflecting film 43, and a broken line indicates areflectance in a region, corresponding to an oscillation region of laserlight, of the reflecting film 43.

As is apparent from this graph, in the case where the refractive indexof the reflecting film 43 is set to a value between the refractive indexof the substrate 11 and the effective refractive index, the relationshipbetween the thickness and the reflectance of the reflecting film 43 issubstantially reversed between in the region corresponding to thesubstrate 11 and in the region corresponding to the oscillation regionof laser light. Accordingly, as shown in FIG. 2, if the reflectance inthe region corresponding to the oscillation region of laser light is setto about 8%, the reflectance in the region corresponding to thesubstrate 11 becomes as large as about 20%. On the contrary, in theexample associated with the related art reflecting film shown in FIG.11, if the reflectance in the region corresponding to the oscillationregion of laser light is set to about 8%, the reflectance in the regioncorresponding to the substrate 11 becomes as small as about 7%. As aresult, according to this embodiment, the reflectance in the region,corresponding to the substrate 11, of the reflecting film 43 becomesnearly three times that in the related art reflecting film, andtherefore, the quantity of stray light entering the semiconductor laserfrom the region corresponding to the substrate 11 becomes nearlyone-third that in the related art reflecting film.

The reflecting film 43 may be configured to have a single layerstructure or a multi-layer structure; however, it may be desirable toadopt the single layer structure from the viewpoint of simplicity offilm formation. The optical thickness L of the reflecting film 43 ispreferably set to a value of λ/4n where n is the refractive index of thereflecting film 43 and λ is an emission wavelength.

According to this embodiment, the reflecting film 43 is preferably madefrom aluminum nitride. The reason for this is as follows: namely, therefractive index of the reflecting film 43 made from aluminum nitridecan be set to a nearly intermediate value between the refractive indexof the substrate 11 and the effective refractive index. Also, since thethermal expansion coefficient of the reflecting film 43 made fromaluminum nitride is closer to that of the semiconductor layer stack 20,it is possible to reduce stress applied to the resonator end surface 41and hence to suppress deterioration thereof, and since the thermalconductivity of the reflecting film 43 made from aluminum nitride ishigher, it is possible to suppress temperature rise at the resonator endsurface 41. Further, since oxide or oxygen is not brought into contactwith the resonator end surface 41, it is possible to prevent acatastrophic optical damage (COD) of the resonator end surface 41 due tooxidation.

On the other hand, the reflecting film 44 has a multi-layer structure(for example, six layers) in which silicon oxide films and titaniumoxide (TiO₂) films are alternatively stacked.

The above-described semiconductor laser can be produced as follows:

A substrate 11 made from sapphire and having a thickness of about 400 μmis prepared, and the following layers are sequentially grown on thec-plane of the substrate 11 by a MOCVD process: a buffer layer 21 madefrom undoped GaN, an n-side contact layer 22 made from n-type GaN, ann-type cladding layer 23 made from n-type AlGaN mixed crystal, an n-typeguide layer 24 made from n-type GaN, an active layer 25 made from GaInNmixed crystal, a p-type guide layer 26 made from p-type GaN, a p-typecladding layer 27 made from p-type AlGaN mixed crystal, and a p-sidecontact layer 28 made from p-type GaN.

In the MOCVD process, trimethyl gallium ((CH₃)₃Ga) is used as a sourcegas of gallium, trimethyl aluminum ((CH₃)₃Al) is used as a source gas ofaluminum, trimethyl indium ((CH₃)₃In) is used as a source gas of indium,and ammonia (NH₃) is used as a source gas of nitrogen; and further,monosilane (SiH₄) is used as a source gas of silicon, andbis(cyclopentadienyl)magnesium ((C₆H₅)₂Mg) is used as a source gas ofmagnesium.

Parts of the p-side contact layer 28, p-type cladding layer 27, p-typeguide layer 26, active layer 25, n-type guide layer 24, n-type claddinglayer 23, and n-side contact layer 22 are sequentially etched, to exposethe n-side contact layer 22 to the outside. Subsequently, a mask (notshown) is formed on the p-side contact layer 28, and parts of the p-sidecontact layer 28 and the p-type cladding layer 27 are selectively etchedusing the mask, to form each of the upper portion of the p-type claddinglayer 27 and the p-side contact layer 28 into a narrow strip shape.

An insulating film 31 made from silicon oxide is formed over the exposedsurface by a vapor-deposition process. An opening is formed in theinsulating film 31 at a position over the p-side contact layer 28, toexpose the surface of the p-side contact layer 28, and an opening isformed in the insulating film 31 at a position over the n-side contactlayer 22. Titanium (Ti), aluminum (Al), platinum, and gold aresequentially deposited through the n-side contact layer 22 side openingand alloyed, to form an n-side electrode 32. Further, palladium,platinum, and gold are sequentially deposited on the exposed p-sidecontact layer 28, to form a p-side electrode 33.

The substrate 11 is polished into a thickness of about 90 μm. Thesubstrate 11 thus polished is divided into parts each having a specificwidth in the direction perpendicular to the resonator direction A, toform resonator end surfaces 41 and 42 for each of the divided parts. Areflecting film 43 is formed on the resonator end surface 41 and areflecting film 44 is formed on the resonator end surface 42 by an ECR(Electron Cyclotron Resonance) sputtering system. After formation of thereflecting films 43 and 44, the substrate 11 is divided into parts atspecific positions in the direction parallel to the resonator directionA. The semiconductor laser shown in FIG. 1 is thus obtained.

The operation of the semiconductor laser thus obtained will be describedbelow.

In this semiconductor laser, when a specific voltage is applied betweenthe n-side electrode 32 and the p-side electrode 33, a current isinjected in the active layer 25, to cause light emission due toelectron-hole recombination. The light thus emitted is reflected andthereby amplified between the reflecting films 43 and 44, to cause laseroscillation. The oscillated light is then emitted to the outside aslaser light mainly through the reflecting film 43. By the way, in thecase where the semiconductor laser is used while being contained in apackage, part of the laser light thus emitted is reflected in thepackage and is returned to the semiconductor laser as stray light. Inthis case, according to this embodiment, since the refractive index ofthe reflecting film 43 against an emission wavelength is set to a valuebetween the refractive index of the substrate 11 and the effectiverefractive index, the reflectance in the region, corresponding to thesubstrate 11, of the reflecting film 43 becomes higher. As a result, itis possible to reduce the quantity of stray light entering thesemiconductor laser from the region, corresponding to the substrate 11,of the reflecting film 43, and hence to suppress occurrence of noise andimprove characteristics such as a variation in output.

According to the semiconductor laser in this embodiment, since therefractive index of the reflecting film 43 against an emissionwavelength is set to a value between the refractive index of thesubstrate 11 and the effective refractive index, the reflectance in theregion, corresponding to the substrate 11, of the reflecting film 43 canbe made higher. As a result, it is possible to suppress stray lightentering the semiconductor laser. This makes it possible to preventoccurrence of noise and to improve the characteristic of thesemiconductor laser.

In particular, in the case of using the reflecting film 43 made fromaluminum nitride, the reflectance in the region, corresponding to thesubstrate 11, of the reflecting film 43 can be made higher, and furtherthe deterioration of the resonator end surface on the reflecting film 41side can be suppressed, and the catastrophic optical damage of theneighborhood of the resonator end surface on the reflecting film 41 sidedue to oxidation can be prevented.

While the preferred embodiment of the present invention has beendescribed, such description is for illustrative purposes only, and it isto be understood that changes and variations may be made withoutdeparting from the scope of the present invention. For example, thematerial for forming the reflecting film 43 is not limited to thatdescribed in the embodiment, and may be another material insofar as ithas a refractive index against an emission wavelength between therefractive index of the substrate and the effective refractive index andfurther it does not absorb laser light.

While the substrate 11 is made from sapphire in the above embodiment, itmay be made from another material.

In the above embodiment, the present invention is applied to thesemiconductor laser including the stack 20 of semiconductor layers eachof which is made from a nitride based group III-V compoundsemiconductor; however, the present invention can be applied to asemiconductor laser including a stack of semiconductor layers each ofwhich is made from another semiconductor such as a group III-V compoundsemiconductor or a group II-VI compound semiconductor.

In the above embodiment, the refractive index of the reflecting film 43against an emission wavelength is set to be larger than the refractiveindex of the substrate 11 and smaller than the effective refractiveindex; however, the refractive index of the reflecting film may be setto be smaller than the refractive index of the substrate and higher thanthe effective refractive index. Even in this case, the same effect asthat described above can be obtained.

In the above embodiment, the present invention is applied to thesemiconductor laser having a structure in which the n-side contact layer22, n-type cladding layer 23, n-type guide layer 24, active layer 25,p-type guide layer 26, p-type cladding layer 27, and p-side contactlayer 28 are sequentially stacked; however, the present invention can beapplied to a semiconductor laser having another stacking structure. Forexample, the n-type guide layer 24 and the p-type guide layer 26 may beomitted, and a crystal deterioration preventive layer may be providedbetween the active layer 25 and the p-type guide layer 26. The currentconstriction may be configured by a structure other than the structurethat the p-side contact layer 28 is formed into a narrow strip. Thesemiconductor laser may be of a reflectance waveguide type or gainwaveguide type.

In the above embodiment, the n-type semiconductor layer is taken as thefirst conductive semiconductor layer and the p-type semiconductor layeris taken as the second conductive semiconductor layer; however, thep-type semiconductor layer is taken as the first conductivesemiconductor layer and the n-type semiconductor layer is taken as thesecond conductive semiconductor layer.

In the above embodiment, the non-light emission side reflecting film 44is provided on the other resonator end surface 42; however, a reflectingfilm having another configuration may be provided on the other resonatorend surface or no reflecting film may be provided thereon.

According to the semiconductor laser of the first present invention,since the refractive index of the reflecting film on the light emissionside against an emission wavelength is set to a value between therefractive index of the substrate and the effective refractive index,the reflectance in the region, corresponding to the substrate, of thereflecting film can be made higher, with a result that it is possible tosuppress stray light entering the semiconductor laser. This makes itpossible to prevent occurrence of noise and to improve thecharacteristic of the semiconductor laser.

Hereinafter, a preferred embodiment of the second present invention willbe described with reference to the drawings. FIG. 3 is a schematicsectional view illustrating a structure of a semiconductor laseraccording to the embodiment. In the semiconductor laser according tothis embodiment, a sapphire (Al₂O₃) substrate is used as a translucentsubstrate 101. The translucent substrate 101 may be configured as agallium nitride (GaN) substrate.

The semiconductor laser has a structure having a light emission functionlayer stack formed on one plane of the sapphire substrate 101, whereintwo electrodes having different polarities are formed on the lightemission function layer stack side, and a light leakage preventive filmR1 is formed on the other plane of the sapphire substrate, that is, onthe surface, opposed to the light emission function layer stack, of thesapphire substrate. The light emission function layer stack is formed bysequentially stacking a buffer layer 102, an n-side contact layer 103,an n-type cladding layer 104, an active layer 105, a p-type cap layer106, a p-type cladding layer 107, and an p-side contact layer 108.

The buffer layer 102, which is for enhancing adhesiveness with thesapphire substrate 101, is made from undoped GaN. The n-side contactlayer 103 is an n-type semiconductor layer having a high carrierconcentration for ensuring an ohmic contact with the n-side electrode,and is made from GaN.

Each of the n-type cladding layer 104 and p-type cladding layer 107functions to confine light in the active layer 105, and is required tohave a refractive index lower than that of the active layer 105. Each ofthe n-type cladding layer and p-type cladding layer 107 is made fromAlGaN.

The active layer 105 is a semiconductor layer for generating lightemission by recombination of electric charges injected in the lightemitting device as a current with holes. The active layer 105 is madefrom undoped InGaN.

The p-side contact layer 108 is a p-type semiconductor layer having ahigh carrier concentration for ensuring ohmic contact with the p-sideelectrode 112, and is made from GaN.

Each of an upper portion of the p-type cladding layer 107 and the p-sidecontact layer 108 has a straight stripe ridge shape extending in onedirection. In the figure, reference numeral 109 designates a ridgeportion composed of the upper layer portion of the p-type cladding layer107 and the p-side contact layer 108. The ridge portion 109 has auniform width W in the resonator length direction.

The n-type cladding layer 104, the active layer 105, the p-type caplayer 106, and a lower layer portion of the p-type cladding layer 107have a specific mesa shape extending in one direction. In the figure,reference numeral 110 designates the mesa portion.

An SiO₂ current constriction layer 111, which does not absorb light fromthe active layer 105, is formed on both side of the ridge portion 109,to form a current constriction structure. The SiO₂ current constrictionlayer 111 is also provided on a side surface of the mesa portion forpreventing short-circuit between the electrodes.

With the formation of the SiO₂ current constriction layer 111 on boththe sides of the ridge portion 109, a stepped refractive indexdistribution in which the refractive index of the ridge portion 109 ishigher and the refractive indexes of both sides of the ridge portion 109are lower is formed in the direction parallel to the junction.

A p-side electrode 112 configured as an Ni/Pt/Au electrode is providedon the p-side contact layer 108 and the SiO2 current constriction layer111 on the light emission function layer side, and an n-side electrode113 configured as a Ti/Al electrode is provided on the n-type contactlayer 103 adjacent to the mesa portion 110.

The light leakage preventive film R1 formed on the side, opposed to thelight emission function layer stack, of the sapphire substrate 101 has amulti-layer structure in which dielectric thin films made from SiO₂ eachhaving a thickness of λ/4n and dielectric thin films made from TiO₂ eachhaving a thickness of μ/4n are stacked in this order, where λ is awavelength of laser light generated in the light emission function layerstack and n is a refractive index of the dielectric thin film.

The formation of the light leakage preventive film R1 is effective tosuppress light emitted from the active layer 105 from being leaked tothe outside through the sapphire substrate 110 as the translucentsubstrate.

FIG. 4 is a graph illustrating a relationship between a current given tothe semiconductor laser and the output of light. In this graph, a curve(a) shows data for a semiconductor laser provided with the light leakagepreventive film R1 and a curve (b) shows data for a related artsemiconductor laser provided with no light leakage preventive film. Asis apparent from FIG. 4, the provision of the light leakage preventivefilm R1 is effective to reduce the quantity of spontaneous emissionlight (see moderately tilted portions of the curves (a) and (b) in FIG.4).

As the number of layers of the dielectric thin films made from SiO₂ andTiO₂ becomes larger, the reflectance of the light leakage preventivefilm becomes higher, and thereby the light leakage preventive effectbecomes higher. The reflectances of the light leakage preventive filmshaving the dielectric thin films of the numbers of two, four, six, andeight are shown in FIGS. 5, 6, 7, and 8, respectively. In thisembodiment, the number of the dielectric thin films is specified at sixfrom the viewpoint of effect and practical usability (easy production).

The thickness of each of the layers of the dielectric thin film is notlimited to λ/4n but may be set to any value insofar as the combinationof the layers having respective thicknesses is effective to increase thereflectance.

The material of the dielectric thin film is not limited to SiO₂ and TiO₂but may be any other material.

In place of formation of the dielectric thin films, metals such as Tiand Al may be formed by a vapor-deposition process, to form a filmcapable of reflecting light which has leaked from the light emissionfunction layer stack. In this case, the Ti layer having a thickness ofabout 10 nm is formed as a buffer layer against the sapphire substrateand then the Al layer having a thickness of about 100 nm is formedthereon.

A semiconductor film capable of absorbing spontaneous emission lightsuch as an InGaN film or an Si film or an insulating film having a largelight absorption coefficient such as TiOx may be used as the lightleakage preventive film. The formation of such a film to a thickness ofabout 100 to 300 nm is effective to suppress a light component leaked tothe outside from the sapphire substrate.

A paint having a color capable of absorbing spontaneous emission lightmay be applied on the surface, opposed to the light emission functionlayer stack, of the sapphire substrate. In this case, the sapphiresubstrate provided with the light emission function layer stack may bedivided into parts, each of which is assembled in a package, and then apaint is applied on the other surface of the sapphire substrate of thepart; or a paint may be applied overall on the other surface of thesapphire substrate before the sapphire substrate is divided into parts.

Another embodiment of the second invention will be described below. FIG.9 is a schematic sectional view illustrating this embodiment. In thissemiconductor laser, an n-type GaN substrate is used as a translucentsubstrate 201. A light emission function layer stack is formed on oneplane of the substrate 201. The light emission function layer stack isformed by stacking a buffer layer 202, an n-side contact layer 203, ann-type cladding layer 204, an active layer 205, a p-type cap layer 206,a p-type cladding layer 207, a p-type cap layer 208, and p-type caplayer 209. A p-side electrode 211 is formed on an upper layer of thep-type cap layer 209, and an n-side electrode serving as light leakagepreventive film R2 is formed on the side, opposed to the light emissionfunction layer stack, of the n-type GaN substrate 201.

The buffer layer 202 is made from n-type GaN, the n-side contact layer203 is made from AlGaN, the n-type cladding layer 204 is made from GaN,the active layer 205 is made from GaInN base, the p-type cap layer 206is made from AlGaN, the p-type cladding layer 207 is made from GaN, thep-type cap layer 208 is made from AlGaN, and the p-type cap layer 209 ismade from GaN.

The p-type cap layer 209 has a ridge structure and an n-type AlGaNburied layer is formed around the p-type cap layer 209.

In the semiconductor laser according to this embodiment, since then-type GaN substrate 201 is used, the p-side electrode 211 and then-side electrode serving as the light leakage preventive film R2 areformed on the front and back surfaces with the n-type GaN substrate 201put therebetween. In particular, since the n-side electrode serving aslight leakage preventing film R2 is formed on the side, opposed to thelight emission function layer stack, of the n-type GaN substrate 201,even if light (wavelength: typically 400 nm) generated from the activelayer 205 passes through the n-type GaN substrate 201, it can beprevented from being leaked to the outside by the n-side electrodeserving as the light leakage preventive film R2.

The n-side electrode serving as the light leakage preventive film R2 isconfigured as an Al film formed by a vapor-deposition process, which iscapable of reflecting light having passed through the n-type GaNsubstrate 201. The thickness of the n-side electrode serving as lightleakage preventive film R2 may be set to a value of λ/4n where λ is awavelength of laser light emitted from the active layer 205 and n is arefractive index of the film. The n-side electrode serving as lightleakage preventive film R2 having the thickness specified as describedabove is effective to perform desirable light reflection at a highefficiency.

The n-side electrode serving as light leakage preventive film R2 may bemade from a material having a conductivity and a light absorptionability. In this case, it is possible to absorb light having passedthrough the n-type GaN substrate 201 and hence to suppress leakage oflight to the outside.

As described above, the second invention exhibits the following effects:namely, even when the semiconductor laser includes a translucentsubstrate, a spontaneous emission light component having passed throughthe substrate can be suppressed, so that the noise characteristic of aproduct using the semiconductor laser can be improved. Further, sincethe light leakage preventive film is directly formed on the translucentsubstrate, it is possible to suppress leaked light irrespective of therelationship with the light emission function layer stack and withoutexerting any effect on the laser characteristic.

The combination of the second invention with the first invention canexhibit a higher effect. For example, as shown in FIG. 10, the lightleakage preventive film R1 may be provided on the substrate of thesemiconductor laser according to the first invention.

What is claimed is:
 1. A semiconductor laser including a substrate,semiconductor layers stacked on said substrate, and a pair of resonatorend surfaces opposed to each other in the direction perpendicular to thestacking direction, said semiconductor laser comprising: a lightemission side reflecting film formed on one of said resonator endsurfaces; wherein a refractive index of said reflecting film against anemission wavelength of laser light is set to a value between aneffective refractive index and a refractive index of said substrate. 2.A semiconductor laser according to claim 1, wherein each of saidsemiconductor layers is made from a nitride based group III-V compoundsemiconductor containing at least one kind of group IIIA elements and atleast nitrogen of group VA elements.
 3. A semiconductor laser accordingto claim 1, wherein said substrate is made from sapphire.
 4. Asemiconductor laser according to claim 1, wherein said substrate is madefrom gallium nitride.
 5. A semiconductor laser according to claim 1,wherein said reflecting film contains at least one kind of aluminumnitride, zirconium oxide, and silicon oxynitride.
 6. A semiconductorlaser according to claim 1, wherein said substrate is a translucentsubstrate, said translucent substrate having a light leakagepreventative film thereon.